1. Field of the Invention
The present invention relates generally to a circuit and a method for outputting a refresh execution signal to a memory cell of a memory device in an auto-refresh mode.
2. Description of the Related Art
Semiconductor memory devices are devices in which data can be stored and from which stored data can be retrieved. Semiconductor memory devices can be classified into random access memory (RAM) and read only memory (ROM). RAM is a volatile memory that needs power supply to retain data, while ROM is a nonvolatile memory that can retain data even when power is not supplied.
A well-known example of RAM is a dynamic RAM (DRAM). A DRAM memory device often comprises a large number of memory cells, each of which is constructed of one transistor and one capacitor. A DRAM memory device achieves storage of data by storing charges in capacitors. Because a charge stored in a capacitor slowly leaks over time, the memory cells of the DRAM memory device need to be periodically refreshed or updated to a full value.
Several schemes have been developed to systematically refresh memory cells in a DRAM memory device, one of which is an auto-refresh scheme. The auto-refresh is a mode in which a memory cell corresponding to an address generated by an internal address counter performs a refresh operation in a fixed period when an auto-refresh command is input. After the refresh operation, the address counter is re-initialized and is provided for the next auto-refresh command.
In the auto-refresh operation, the refresh cycle is controlled by the command from outside. In response to the command, the memory device performs an auto-refresh operation. Therefore, the period of time that it takes to refresh all of the memory cells in the memory device cannot be adjusted. Since the DRAM memory device generally consumes a relatively large amount of power when the memory cells of the DRAM are being refreshed, it is desired to provide a method and a circuit for outputting a refresh execution signal according to different conditions so as to reduce current consumption of the memory device.